The present invention relates to a process for the preparation of esters of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid and particularly it relates to a process for the preparation of esters of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid by enzymatic transesterification of enantiomeric mixtures.
The esters of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid or (2R,3S)-2,3-epoxy-3-(4-methoxyphenyl)-propionic acid are intermediates useful for the synthesis of compound (+)-(2S,3S)-3-acetyloxy-5-[2-(dimethylamino)-ethyl]-2,3-dihydro-2-(4-methoxyphenyl)-1,5-benzo-thiazepin-4(5H)-one, a drug with coronary vasodilating activity known with the name of Diltiazem (Merck Index, XI Ed., No. 3188, page 505).
The preparation of Diltiazem, starting from esters of 3-(4-methoxyphenyl)-glycidic acid, can be carried out according to several methods in the literature.
Examples are reported in the British patent No. 1,236,467, in European patents No. 127882 and No. 158340 and in British patent application No. 2,167,063, all in the name of Tanabe Seiyaku Co. Ltd. In order to prepare Diltiazem is necessary to carry out an optical resolution.
It is clear to the man skilled in the art that it is economically more convenient to carry out a resolution step at an early stage of the process since the economic value of the product on which the resolution is carried out is lower and consequently the undesired isomer has a lower cost.
Therefore, it is advantageous to have the esters of 3-(4-methoxyphenyl)-glycidic acid in an enantiomerically pure form since such compounds are the first optically active intermediates of the synthesis.
Several methods for the preparation of esters of 3-(4-methoxyphenyl)-glycidic acid in enantiomerically pure form are known.
Most of these methods foresee the resolution of a racemic mixture of 3-(4-methoxyphenyl)-glycidic acid with optically active bases and the subsequent esterification of the optically active acid (Japanese patent application No. 61/145160 in the name of Nippon Chemiphar Co. Ltd.; C.A. 106:32600u).
However, the difficult industrial application of such resolution methods is known. In fact, it is necessary to carry out the separation, the isolation and the purification of the diastereoisomeric salts under controlled conditions and there is the need of recovering the generally quite expensive optically active base.
Moreover, in the specific case, there is the problem of the high instability of 3-(4-methoxyphenyl)-glycidic acid which can cause serious troubles during the various steps of the resolution process. Enzymatic resolutions of esters of various structure are generally known (Angew. Chem. Int. Ed. Engl., 24, 617, 1985 and 28, 695, 1989).
However, as far as we know, enantioselective enzymatic transesterifications of esters of 3-(4-methoxyphenyl)-glycidic acid or of its analogous have never been described.
We have now found and it is the object of the present invention a process for the preparation of esters of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid of formula 
wherein R is a linear or branched C1-C8 alkyl group; a C5-C6 cycloalkyl group or a 2,2-dimethyl-1,3-dioxolane-4-methyl group;
which consists in subjecting to enantioselective enzymatic transesterification a mixture of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid methyl ester or ethyl ester (I, Rxe2x95x90CH3, C2H5) and its (2S,3R)-enantiomer (ent-I), by using an alcohol which is different from the alcohol esterifying compound I and ent-I and which is selected among a linear or branched C2-C8 aliphatic alcohol, a C5-C6 cycloaliphatic alcohol or 2,2-dimethyl-1,3-dioxolane-4-methanol, optionally in the presence of a suitable solvent or mixture of solvents, and in the separation of the transesterified ester from the untransesterified one.
The process object of the present invention allows to prepare intermediates useful for the synthesis of compounds with coronary vasodilating activity.
The enzymes useful for the transesterification reaction can be of different nature.
In particular, lipases of animal or microbial origin or proteolytic enzymes such as for example xcex1-chymotrypsin can be used.
Among the lipases of animal origin useful in the process of the present invention, pig liver and pig pancreas lipases may be cited. Among the lipases of microbial origin, lipases from Candida, Mucor. Pseudomonas and Aspergillus microorganisms may be cited.
Examples of suitable alcohols are ethanol, n-propanol, 2-propanol, n-butanol, 2-butanol, 2-methyl-2-propanol, n-pentanol, 2-pentanol, 3-pentanol, n-hexanol, n-heptanol, 2-heptanol, n-octanol, 2-octanol, cyclohexanol, cyclopentanol and 2,2-dimethyl-1,3-dioxolane-4-methanol.
In particular n-butanol, 2-butanol, cyclohexanol, n-octanol and 2,2-dimethyl-1,3-dioxolane-4-methanol are the preferred alcohols.
The lipases and the proteolytic enzymes act on enantiomerically opposite substrates.
In particular, in the enantiomeric mixture of compound I and ent-I (R=methyl, ethyl) the pancreatic enzyme, xcex1-chymotrypsin, transesterifies the ester with the desired (2R,3S)configuration, that is compound I, while the lipase transesterifies the (2S,3R)-enantiomer, that is compound ent-I.
The selection of the transesterifying agent (alcohol) to be used depends on the nature of R (methyl or ethyl) in the starting enantiomeric mixture.
In fact, according to the process object of the present invention, only one enantiomer transesterifies while the other remains unchanged.
Consequently, in order to separate the transesterified ester from the untransesterified at the end of the transesterification reaction, when R=methyl, all the above listed alcohols can be used, while when R=ethyl all the higher homologous of ethanol can be used. The transesterification reaction is carried out by contacting the enantiomeric mixture of compound I and ent-I (R=methyl, ethyl) with the enzyme and with the suitable alcohol.
Alternatively, the enzyme can be immobilized on suitable supports according to conventional techniques.
Examples of suitable supports are absorbent resins, acrylate polymers, porous materials, agarose or celite.
Preferably, a further suitable solvent or mixture of solvents such as for example hexane, cyclohexane, toluene, benzenes methyl ethyl ketone, diethyl ether is used if the transesterification reaction is carried out with the lipase enzyme.
At the end of the transesterification reaction, the two esters are separated according to known techniques.
For example, crystallization, chromatography or extraction with a suitable solvent or mixture of solvents may be used.
Examples of suitable solvents for the extraction are hexane or its mixtures with ethyl acetate, methanol and acetonitrile.
The operative conditions of the transesterification reaction are those normally used during the enzymatic reactions.
Such conditions take into account the pH and the temperature range in which each enzyme performs.
Generally such ranges are comprised between 6-11 pH units and between 0xc2x0 C. and 70xc2x0 C. respectively.
Preferably, the process of the present invention is carried out at a pH comprised between 6 and 8 and at a temperature comprised between 20 and 60xc2x0 C.
At the end of the process the enzymes retain most of their activity and consequently they can be used again for several cycles.
Preferably, due to the easy availability on the market at lower cost, lipases of microbial origin and, in particular, lipases from Candida Cylindracea or xcex1-chymotrypsin are used in the process object of the present invention.
The process object of the present invention allows to prepare the compounds of formula I with good yields and high enantiomeric purity and to recover also the undesired enantiomer.
It is possible, therefore, to carry out its racemization or inversion of configuration in order to increase the global yields of the process.
Moreover, the used enzyme retains its enzymatic activity and, consequently, it can be used again for several times.
If desired, compound I can be further purified by crystallization.
As regard such feature, we have surprisingly found that when the compounds of formula I have at least about 80:20 enantiomeric ratio, their crystallization provides compounds I with a higher enantiomeric purity and this is independent from the source of the mixture. In particular, starting from the compounds of formula I with about 80:20 enantiomeric ratio, a 95:5 enantiomeric ratio is obtained by a simple crystallization.
Solvents suitable for the crystallization are lower alcohol such as for example methanol, ethanol, propanol, butanol.
Therefore, it is a further object of the present invention a process for increasing the enantiomeric purity of esters of (2R,3S)-3-(4-methoxyphenyl)-glycidic acid having at least an 80:20 enantiomeric ratio, which consists in crystallizing such esters with a suitable solvent.